Subscriber transmission system



March 23, E954 P. E. REEVES SUBSCRIBER TRANSMISSION SYSTEM 2 Sheets-Sheet l Filed June 5, 1948 NTN 5.252% :95am S mism mz3 d@ k N mor om .w M

mm mix om\ ,/Lm mmm 52m@ m ESE .ruim dior( PIERCE E. REEVES JNVENTOR.

H/s AGE/vr I March 23, 1954 E. E. REEVES SUBSCRIBER TRANSMISSION SYSTEM 2 Sheets-Sheet 2 r-33 Filed June 5, 1948 FIG2 TIME

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FIGS

VIDEO SIGNAL 59 INPUT PIERCE E. REEVES .INI/ENTOR.

Patented Mar. 23, 1954 UNITED star it PATENT OFFICE SUBSCRIBER TRANSMISSION SYSTEM Pierce- E. Reeves, Park Ridge, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois Application .lune 5, 1948, Serial No. 31,345

Claims.

, tervals the video portion of a transmitted television signal'is delayed a predetermined amount with .respectl to the synchronizing signal and blanking pedestal positions thereof. The systemfdi'sclosed in the above mentioned patent provides for the transmission of a key signal to subscriber'receiving systems to indicate the time intervals during which the video portion or" the television signal is so delayed so that corrective circuits may be initiated. in such receiving systems lto cause proper reproduction of the television/signal. It was found in the above described s'ystem that when the television signal was corrected in a subscriber receiving system an objectionable flicker occurred in the reproduced image'.` This condition was due to the fact that the transmitted television signal contained a video portion, the timing of which was changed between a normal and a delayed value during spaced intervals, and the television signal further contained blanking pedestals which remained fixed in time. The signal content of each line of y the video portion of the transmitted signal changed therefore, 'each time this portion varied between a normal and delayed condition, which gave rise to the above mentioned flicker in the reproduced image.

vIt is,"accordingly, an object of the present invention to provide an improved subscription television'transmitter for transmitting a. coded television signal which induces additional? signal gomponents, for eliminating distortion that might otherwise arise.

'More particularly, it is an object of this invention to provide an image transmission system in which a video blanking circuit is incorporated, which circuit blanks the video signal for certain time intervals, the timing of these intervals changing inl synchronism with the change of timing of the video portion, in such a manner that the4 video signal content during each line interval does not'change when the timing of this portion is altered from one state to another, and whereby the above mentioned objectionable flicker is removed-from the ytransmitted television signal.

It is another object of the invention to provide a novel method of operating a subscription television transmitter.

The features of this invention, which are be lieved to be new, are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by reference to the accompanying drawings, in which:

Figure 1 shows an image transmission system of the subscriber type incorporating the present invention,

Fiugre 2 shows various diagrams useful for the understanding of the invention, and

Figure 3 shows in detail the proposed video blanking circuit.

The system shown in Figure 1 is substantially' similar to the system disclosed in the previously mentioned patent, and this system includes a television camera I of the usual type. Camera I contains a lens system 2 for focusing an image upon an iconoscope or image orthicon 3, which tube is provided with the usual vertical scanning coil 4 and horizontal scanning coil 5. Video signals developed by the i'conoscope 3 are amplified through video amplifier 6 and are subsequently transmitted through a video blanking stage 7 to a synchronizing vsignal and pedestal mixer stage 8, in which mixer appropriate pedestals'and synchronizing signals "are added to the video signals to produce the desired television signal. The

television signal is then transmitted to a background reinsertion device 9 and to a carrier wave generator and modulator ID in vwhich the television signal, appropriately adjusted as to background level, is modulated on a. carrier wave which is radiated from antenna I I.

Vertical and horizontal synchronizing signals are generated in vertical and horizontal synchronizing generators I2 and these synchronizing signals are impressed on mixer 8 by way of respective leads I3 and I4. Vertical synchronizing signals from generators I2 are further impressed on vertical sweep generator l5 by way of leads IS, and the sweep signal output of generator I5 is applied to the vertical sweep coil 4 of tube 3. Horizontal synchronizing signals from generators I2 are further applied to delay line and switch II by way of leads I8, the'output signals from delay line and switch I'I being impressed by Way of leads I9 on a horizontal sweep generator 2i! of normal construction, and the sweep signal output of generator 20 being applied to the horizontal sweep coil 5 of tube 3.

In order to perform the desired alterations of the horizontal synchronizing signalsl in the delay and issued March 11, 1952, as Patent No. 2,588,4153;v v

and assigned to the present assignee. Such lower.-

frequency pulses are impressed'thrgoughconlu n tors`23 on a key frequency oscillator 24 which produces bursts of oscillation or key signal in re,-

sponse to each pulse on conductors 23, the-frequency of which oscillation being suitablefpi1 transmission by wire line conductor to subscriber receiving systems.A These bursts of oscillation are impressed'on conductors 25 whichv are connectedto such receiving systems by way of phone lines, power. lines or. the like..

The, key signal which appears at various time.-

intervalsfonconductors 25 is applied to key frequency filter and switch operator 26` by way o|f leads 21,; Stage 26j isjfurther supplied with positire.. 'going' vertical synchronizing pulses from generatorsV I2; by Way of leads 28, rThis stagein tho. nrosonoe of the key Signal on conductors4 2.1, utilizes the positive going vertical pulses on conductors 2,8, to, change the respective potential values'of leadsl 2 ,9 and 3B with respect tolead 3|.. Such change4 inthe potential values of the above mentioned leads Icauses delay line and l Switch. I7 to. alter the. timing of the horizontal synch i',onizvingy pulses passing through leads I8,to I 9' from, one condition to another correspondingly, toalter' the timing of the video signal gen.- franchi tube. o,

The system thus far described is similar to, thatA disclosed in previously mentioned Patent 2,5fl '7,594 andasin that patent the key signal is gen ratedi by oscillator 24 at various time in,-v telfvals, `vvlfiich intervals are: dependent upon the times of occurrence of vpulses on leads 23, and4 this key, signal causes delay line and switch Il tobe operated to delayr the video portion of.

theltransmitted television signal a.v predetermined amount during these time intervals. It canfurther be seen that the key signal is transmitted to` subscriber receiving systems to indicate the. timcsoi occurrence of the delayed video signal,` sothat corrective circuits may be initiated at ,the subscriber receivers.

As previously stated, it was found with such a, systemthatan objectionable iiicke-r occurred inthecorrected image at the subscriber receiver duel-tothe, factthat the blanking pcdestals remain `fixedin u'tirne to prevent unauthorized syn.-` chronization thereon, while the video was moved from anormalto a delayed position in the trans mitted ,signlallfcr coding p-urposes. The present., system provdesadditional stages for inclusion the above. .de scribe`d.system, by means of which tl`1`e`,.vlideo.signal/generated by tube 3 is blanked for44 certain Y intervals, the timing of which in. teryals ,changes insynchronism with the change inlthetiming of the video portionoi the tele-a visionsignal. For this purpose, a blanking pulseA generator, 32isconnected to leads I9, this gen-`` erator may be a usual multivibrator o-r blocking oscillator, and is triggered by the horizontal synchronizing pulses from delay line and switch I1. l".lhese horizontal synchronizingY pulses., are.

delayed at certain time intervals and corresponding pulses at the output of generator 32 are similarly delayed. The output of generator 32 is applied to video blanking stage l, which stage may be adjusted to blank the video signals passing therethrough at times corresponding tothe timesA of occurrenceof,.thel pulse.l output of generator 32.

Referring now to Figure 2, horizontal synchronizing pulses 33 superimposed on blanking pedestals 34 are represented at A. The video portion offthe signal, not shown, Wo-uld appear between these'v blanking pedestals, and as this portionjsyrnovedback and forth in time with respect to the A4pedestals in accordance with the arrangement of Figure 1, the signal content of the video portionh during each line interval in itsL undelayed state is not the same as the signal content of this portion during each line interval in its delayed state., This change in signal content of, the videoportion during the various line intervals givesrise tcnicker in the received tele- C, the pulses shown at, Boccurring at4 the out'-l put of generator 32 when the video is in its undelayed state,l and those shown at C occurringl when the video is in its delayed state. It; can be seen that the p ulsesatB having. a duration equal; to that of pedestalsl 34: of A plus the interval b, similarly the pulses at C have a similar duration, having that of pedestalsd plus interval c. Interval b equalsinterval c and corresponds to the predetermined time delay of the video! duringv the spaced. intervals. It can` further be seen that the pulsesat B and C change their phase relationshipv withthe xedppedestalsat A in synchronism withv the change in the video during these intervals. The pulses shown at B and C cause stage 1 of Figure 1 toblank, the video signal for certain intervals, such blanking beingA shown at D andgE. InA D the videotvnot shown,' is assumed to be in its undelayed state and the pulses shownpat Bpause stage 'Ito blank a portion of the video during intervals t.' which intervals wouldnot be blanked' in, the absence of' these pulses. At E the video, not shown, is assumed to be initsdelayedstate, and the; pulses,

shown at C causeV stage 7 to blank a portion. of the video during time, intervals tf, which intervals similarly would not be blanked inthe absence of the last mentioned pulses.

It is apparent that in the absence of the video blanking during intervals t, when the videoA is in its undelayedstate, the portion of the video that would appear during these time intervals would not appear when the video is` in itsdelayed state, since this portion `Jvould then be blanked by a pedestal 34. Similarly, if not blanked,.the por-7 tion of the video appearing during intervals t'. when ,the video yis, delayed does nctfappear. when the videois initsundelayedstate. Hence, the..v video signall content during each lineinterval.A

ingI blanking2 duringl ,thev interval, t when theV video is undelayed, and shifting this blanking, to intervalt. Whenthe video is delayed.

Thevarious: components ofthe present-,sys-

tom., apart from; Stage, 1,.havof.boono.roviouo1r.

described indetail inthe aforementioned patent',r andlfurther description thereof is believed to be unnecessary. The circuit of the video blanking stage 1 is shown in Figure 3.

`Referring now to Figure 3, this circuit includes av dischargedevice 35. Video signals from image tube 3 of Figure 1 are limpressed across terminals 36;'one of these terminals being grounded and theother'coupled to control electrode of device 35- through capacitor 38, control electrode 37 being connected to ground through resistor 39. The anode 40 of device 35 is connected directly to the positive terminal of a source of unidirectional potential 4| ,and the cathode 42 of this device is connected to the cathode 43 of device 44, cathodes 42 and 43 being connected to ground through common cathode v'resistor 45. The anode t6 of devicef44 is connected to cathode 4l of device 48 through resistor 49, and the control electrode 50 of this device is coupled to control electrode of device 48 through series connected coupling capacitors 52 and 53, control electrode 5e being connected to ground through grid leal; resistor 50'. The anode 54 of device 48 is connected directly tothe positive source lli, and anode it of device 44 is connected to the mid-point or' a potentiometer arrangement which comprises resistors 55, 56, 51 and 58 connected across source 4|,v the sum ofthe values of resistors 55 and 56 being equal to the sum of the values ci resistors 51 and 58. 'I'he junction of resistors 55 and 5t is coupled to ground by capacitor 59 so that resistors 55and`58 may be given a high value for low drain on source 4| and still allow the circuit 55-59 to exhibit low impedance to alternating currents. Control electrode 5| is connected to a movable tap on resistor 5'! 'through resistor 60. Blanking pulses from stage 32 of Figure 1 are impressed across terminals 5|, one of these terminals being connected to ground and the other connected to the common connection of capacitors" 52 and 53. The output signals from this circuit; which are impressed on stage il of Figure 1, arebbtained across terminals 52, one of thesev terminals being grounded and the other conl nectedto anode 4&5 of device 44. -It can'be seen that devices 44 and 43 are connected across source 4| in series, and since the value 'of resistor 55 plus resistor 5B is equal to the value of resistor 51V plus resistor 55, these devices have equal potentials impressed thereacross. Devices 44 andv 48 have similar characteristics, resistor 49 being included in the circuit of cathode 41A to correct any unbalances in lthe system and4 to insure that both the devices 44 and 48 reach n cut-,off at the same point. yIntheabsence of negative going pulses from generator 32 o-f Figure 1 on terminals 6|, a video signal impressed across terminals 36 is applied to the control electrode 3l of device 35. Devices 44 and '48 are arranged-to be in a conductive state when no negative going pulses appear on termitrials 61, and this video signal, by reason ol th-e cathode'coupling between devices 35 and 44, appears aft the anodev i6 of device 44 and, hence, across'terminals 62. It is noted that the signal outputacross terminals 62 has the same phase f as' thev signal impressed across terminals 35, since i device-35 is cathode coupled to` device 44 and,= hen'cel there vis no phase reversal in the signal translating circuit of AFigure 3. The average level l of the output signal at terminals 52 may be varied byy adjusting the tap on resistor 5'! and, hence,- the vbias on controlelectrode 5 essere. scies ,Pulse .Pf ampliada tive amplitude suicient 48, these pulses being applied to the control elecequivalent to the amplitude-of pulses from 'stage .'l

to cut-01T devices 44 and trodes of these devices through coupling capacitors 52 and 53. The potential across the output terminals now becomes substantially one-half that of source 4| due to the connection of these terminals to potentiometer 55-58, and this potential is independent .of the video signal across terminals 36. Hence, the average value of the video signal at output terminals G2 may be given any desired..

value with respect to the blanked level by ad-v Justment of the tap on resistor 57,. the blanked level remaining at one-half the value of source 4|. This average valueis usually adjusted to be substantially equal or a littleV below the blankedv level. It has been found with such anadjustment that the dicker in the received image is completely removed and at the same time the blanked level is sufficiently low to prevent unauthorized synchronization thereon.

It can be seen by reference to Figure 2 that when the pulses at B are impressed on terminals G l, the video blanking extends for time T and has a phase relation with the iixed blanking pedestals, as shown at D. In the intervals between the pulses at B, the video is passed by the system of. Figure 3, and when a shift in the timingof the video causes the pulses at B to change in phase relation with the fixed pedestals to the pulses'at C, these last mentioned pulses are impressed on terminals 6|, and cause the blanking circuit toA blank the video for a similar time interval T this blaming intervai having a phase relation' with the blanking pedestals as shown at E.

This invention therefore provides in a television transmission system ofthe subscriber type, a circuit for insuring that the video portion o-f the transmitted signal has'the same signal content in each line interval when the timing of this portion is altered with respect to the synchronizing- 'and pedestal portions ofthe transmitted signal, and hence this, circuit suppresses undesirable icker which would appear in the received image.

n While a particular embodiment oi' the invention has been shown and described,va'rious modications may be made without departing therefrom. It is intended in the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

I claim: 1

l.. A subscriber television transmitter comprising: a video-signal generating device; a scan-' ning system for controlling said device to develop during `recurrent line-trace intervals a video signal representing a scanned subject and including a synchronizing-signal generator oi' developing during interposed retrace intervalsv a `synchronizing signal normally having acertain f time relation with respect to said video signal; a" coding .circuitiior altering the timing ofisaid'.

video signal with respect to said synchronizing-l ning system for; producing `a-.television signal. which includes alternation said video rsignaland said synchronizing signal'. means coupled. to said mixer for transmitting', said television.

signal to a point' -reni'oit'efrom said transmitter; j` a blanking 'circuitcoum of such pulses, the cir,.

trolled .by said.-coding-1.circuitl for actuating said blanking circuitto blank said video signal nor:- many-during4 av certain preselected portion of:v each. ofv said line-trace intervals, and to blankf said-IvideoA signal during' a. different preselected portion f each of said' line-trace intervals dur'.- ing, saidV spaced operating intervals.

2.` Aisubscriber television transmitter compris-- ing :vv a `video-signalgeneratingdevice; a scanning system-for4 controllingsaid device to developv during` 'recurrent line-trace intervals a video'- 'signal representing. a scanned subject and in-- cluding a synchronizing-signal generator for developing-l during interposed retrace intervals a .synchronizing signal normallyv having a certain circuit ,coupled to lsaid mixer circuit for blankingsaidfvideo signal;v and means controlled by said codingcircuitfor actuating said blanlring circuit to blank said video signal normally at the trailing portion of=.each of said line-trace intervals, and during said spaced operating intervals to' blank said-video' signal at the leading portion of each of said.-linetrace intervals, for a period. corresponding to-the timey delay of said video. signaly with-respect togsaid synchronizing signal during; said spaced intervals.

B. :Aisubscriber television transmitter compris.-

. ing: a'video-signal generating device; a scanning system for controlling said device to develop duringirecurrent-line-trace intervals a video signal representing a scannedsubject and including a synchronizing-signal generator for developing during; interposed retrace intervals a synchronizvingisignalf.normally having a certain time relation.With.respect-.-to said video signal; a coding circuit coupledto said l,scanning system for alter.- 1

ingthe timing of said video lsignal with respect` to-said synchronizing-signal during spaced operating` intervals; a mixer circuit coupled to said'.` device and to said scanning system for producing' a.- television* signalwhich includes in alternation said video signal and said synchronizing signal; means including a carrier-Wave generator and modulator coupled to said mixer for transmitting said televisionv signal to a point remote from said transmitter; a blanking circuit coupled to said mixer circuit for blanking said video signal; and

, means-controlled by said coding circuit for supplyingr blanking pulsesto said blanking circuit to blank said-video signal normally during a certainIpreselectedl portion of each of said lmetrace intervals,- and` to blank said video signal 'duringzaL-different preselected portion of each of-saidrlinetraceintervals during said spaced operating intervals.l

'-4. A-'subscriber television transmitter comprising: a video-'signal generating device; a scanning slystemvforcontrolling said device to developv during recurrent line-trace intervals a video signal representing a scanned subject and including alsynclironizing-signal' generator for developing during interposed retrace intervals a synchronizingsignal .normally having a certain time relation with respect to lsaid video signal; a coding said line-trace intervals, and-for actuating saidcircuit coupled to said scanning system for alter.--

ing the timing of said video signal withrespectto said synchronizing signal during spacedv oper-1 ating intervals; a mixer circuit coupled to said device and to said scanning systemfor producing a television signal which includes in alternation' said video signal and said synchronizingsignal; means including a carrier-Wave gener'- ator and modulator coupled to said mixer :for transmitting said television signal to a-point remote from saidtransmitter; avv blanking cir :uitJ disposed between saidl video-signal generating device and said mixer circuit for suppressing said video signal and supplying a-potential having 'a predetermined amplitude to said mixer circuit;l andmeans controlled by said coding circuit for normally actuating said blanking circuit lfor recurrent intervals including a predetermined-portion-of' eachv of said line-trace intervals, and for 4actuating said blanking circuit for recurrent'- intervals including a different predetermined portion of each of said line-trace-intervals during said spaced` operating intervals.

5. A subscriber television transmitter compris` Iing: a video-signal generating device; a scanning system for controlling said device to ldevelop during recurrent line-trace intervalsa video signal representing a scanned subject and including a synchronizing-.signal generator for developing during interposed retrace intervals av synchronizing signal normally having a'certain time relation with respect to said video signal; a key.- :frequency oscillator coupled to said-'synchronizing-signal generator 'for developing a-lzey signal;-

` a. coding circuitincluded Ain said scanning system. for altering the timing of said yideo vsignalwith respect to said synchronizing-signal; apparatus coupled to said key-frequency .oscillator -for actuating said coding circuit duringr spaced operi-.-

ating intervals; a mixer circuit .coupled-to said device and'to said scanning system Ifor producing a television signal Whichincludes in alternation said'video signaland said synchronizing signal;

means including' a carrier-Wave `*generator andmodulator coupled -to-saidmixer for transmitting. said television signal toafpoint-remote-from said- Vtransmitter; a blankingcircuit kdi spgosed-:betvveen said video-signal-.generatingdevice and saidmixer circuit for suppressing said video'signalas applied to said mixercircuit; and means con:

trolled by said coding circuit for normally-actuating said blanking circuit for recurrent -intervals including a predetermined portion of each-of blanking circuitforrecurrent intervals including a different predetermined portion of each .of said line-trace intervals during said spaced-operating intervals.

`6. In' vasubscriber television-transmitter having a video-signal generating device tor-developing a-video signal during each of-a series Aof'timve spaced linetrace intervals and apparatus for transmitting said-signal to a point remote from said transmitter, a blanking stage for coupling said device lto saidtransmitting apparatus, said blanking ystage comprising a potential-divider network', a normally conductive' electron-discharge devicehaving an input circuit" coupled to said video-signal generating4 device and'having an output electrode coupled to a point on said potential-divider networkwhereby a -portionof said network forms an output circuit' for -saidf dischargeV device, and a connection extending yfrom said point on said v potential divider andha*v point 'of reference' potential to sadtransri'tt-ing "-apparatusfand a source of control pulses coupled to said discharge device for rendering said discharge device non-conductive during a selected part of each of said line-trace intervals to establish the' level of video signal applied to said `transmitting apparatus at a preselected value during said part of each of said line-trace intervals.

7. In a subscriber television transmitter having a video-signal 'generating device for developing a video signal during each of a series of time spaced line-trace intervals and apparatus for vtransmitting said signal to a point remote from 'Y said transmittena blanking stage for coupling said device to said transmitting apparatus, said blanking stage comprising a potential-divider network,` a normally conductive electron-discharge device having an input circuit coupled to lsaid vvideo-signal generating device and having an output 'electrode'coupled to a point on said potential-divider network whereby a portion of vsaid network-forms an output circuit for said discharge device, y a variable-impedance device connected inv shunt with said portion of said potential-divider network, and a connection extending from said point on said potential divider and a point of reference potential to said transmitting apparatus for applying said video signal lto said transmitting apparatus with an average value determined'by the setting of said variableimpedance'device; and a source of control pulses coupled to said discharge device for rendering said discharge device non-conductive during a `selected part of each oi said line-trace intervals 'to said video-signal generating device and having an output electrode coupled to a point on said Ipotential-divider network whereby a portion of said network vforms an output circuit for said discharge device, a second normally-conductive electron-discharge device forming a variable-impedance device connected in shunt with said portion of .said potential-divider network, and a connection extending from said point on said potential divider and a point of reference potential to said transmitting apparatus for applying said video signal to said transmitting apparatus with an average value determined by the setting of said second discharge device; and a source oi 'control pulses coupled to said discharge devices for rendering said discharge devices non-conductive during a selected part of each oi said linetrace intervals to establish the level of said video signal applied to said transmitting apparatus at a preselected value during said part of each of said line-trace intervals.

9. In avsubscriber television transmittal` having a video-signal generating device for developing a video signal during each of a series oi time spaced line-trace intervals and apparatus for transmitting` said signal to a point remote from said transmitter, a blanking stage for coupling blanking stage comprising: a potential-divider network, a first normally conductive electron-discharge device having an input circuit coupled to said video-signal generating device and having an output electrode coupled to a point on said potential-divider network whereby a portion or said network forms an output circuit for said discharge device, a second normally conductive electron-discharge device having a cathode connected to said point on said potential-divider network, an anode connected to one end of said network, and a control electrode coupled to an adjustable tap thereon, and a connection extending from said point on said potential divider and the other end thereof to said transmitting apparatus for applying saidvideo signal to said transmitting apparatus with an average value determined by the setting of said adjustable tap;.and a source of control pulses coupled vt'o said discharge devices ,l for rendering said discharge devices non-conductive during a selected part of each of saidvline-trace intervals to establish the level of said video signal applied to said transmitting apparatus at aprevselected value during said part of each offsaid line-trace intervals.V f Y 10. A subscriber television transmitter comprising: a video-signal generating device; a scanning system fo'r controlling said device to develop during recurrent line-trace intervals a video signal representing a scanned subject and including a synchronizing-signal generator for developing during interposed retrace intervals a synchronizing signal normally having a certain time relation with respect Ato said video signal; a coding circuit for altering the timing of said video signal with respect to said synchronizing land means controlled by-s'aid coding circuit `for actuating said blanking circuit to blank said video signal normally during a certain preselected portion of each of saidl line-trace intervals, and to blank said videov signal during a different preselected portion of each of said line-trace intervals during said spacedoperating intervals. c

11. A subscriber television transmitter comprising: a video-signal generating device; a scanning system for controlling said device to develop during recurrent line-trace intervals a video signal representing a scanned subject and including a synchronizing-signal generator for developing during interposed retrace intervals a synchronizing signal normally having` a certain time relation with respect to said video signal; a coding circuit coupled to said scanning system for altering the timing of said video signal with respect to said synchronizing signal during spaced operating intervals; a mixer circuit coupled to said device and to said scanning systemvfor producing a television signal which includes in alternation said video signal and said synchronizing signal; means including a carrier-wave generator and modulator coupled to said mixer for transmitting said television signal to a point remote from said transmitter; a blanking circuit for establishing the value of said video signal at 1 1 ,A a preselectedreierence level representing a shade "".v'aluewhich is independent of said scanned sub- ",fj't'; and means controlled by said coding circuit y, for normally actuating said blanking circuit for -`"recurrent intervals including a predetermined portion o f each of said line-trace intervals, Aand 4fior .actuating said blanking circuitI for recurrent y fintervals including a different predetermined portion of each of said line-trace intervals during s aid spaced operating intervals. 'o 1, 2.L A subscription television transmitter comprising: a video signalgenerating device; a scan- "'n ing"sy stem for controlling said device to de- "ve'lop during. recurrent line-trace? intervals a `video signal representing a scanned subject and "including a synchronizing signal generator for 'developing asynchronizing signal during each fof recurrent retrace intervals interposed between 'said line-trace intervals; a codingcircuit for altering the time relation of said video signal with respect to said synchronizing signal in accordance with a coding schedule; a mixer circuit coupled v,to saiddevice and to said scanning sysptern for producing a television signal which inlcludes in alternation said video signal and saidv synchronizing signal; a blanking circuit for establishing said video signal at a reference level representing a shade value independent of the scanned'subject; and means controlled by said coding circuit for actuatingl said blanliing circuit l13, A subscription television transmitter comi prising: a video signal generating device; a scanning system forcontrolling said device to develop Vduring'recurrent line-trace intervals a video sig- 'nal representing a scanned subject and including a synchronizing signal generator for developing synchronizing signal during each of recurrent 'retrace intervals interposed between said linevtrace intervals; a coding circuit ior altering the 'time relation of said video signal with respect to j'said synchronizing signal in accordance with a 'coding schedule; a mixer circuit coupled to said device and to saidscanning system for producing Ia ,television signal Which includes in alternation said 'videojsignal' and said synchronizing signal;` a blanking circuit for establishing said video sigrial ata reference level representing a shade value independent of the vscanned subject; and means "for supplying control signals to said blanking circuit duringa selected portion of each of said line-'trace intervals to actuate said blanking circuit and equalize the video content represented 'by' said video signal during said line-trace inamaze? 12 tervals irrespectiyeof the time relation-.ofsaid video signal and .said synchronizing signal.

14. rihe -method of operating a subscription television transmitter which comprises the following steps: scanning a subject during recurrent line-trace intervals todevelop in each such interval a video signal representing the scanned subject; developing during interposed -retrace intervals a synchronizing signal; varying the time relation of said video signal yin respect of said synchronizing signal in accordance with a coding schedule; and establishing a selected portion of said video signal of each such line-trace interval at a reference shade value to equalize the video content represented by said video signal during said line-trace intervals irrespective of the time relation of said video signal and said synchronizing signal.

15. The method of operating a subscription television transmitter which comprises the following steps: .scanning a subject during recurrent line-trace intervals to develop in each such interval a video signal representing the scanned subject; developing during interposed retrace intervals a synchronizing signal; varying the time relation of said video signal in respect of said synchronizing signal in accordance with a coding schedule; and establishing a portion of said video signal of each such line-trace interval, selected in accordance with said coding schedule, at a reference shade value which is independent of the scanned subject to equalize the video content represented by said video signal during said line-trace intervals Yirrespective of the time relation of said video signal and said synchronizing signal.

PIERCE E. REEVES,

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